Diffusely reflective polymer-based optical films are known. For example, U.S. Pat. No. 5,825,543 (Ouderkirk et al.) and U.S. Pat. No. 7,057,816 (Allen et al.) describe, among other things, optical films with a disperse phase of polymeric particles disposed within a continuous birefringent matrix of another polymeric material. The films are oriented, typically by stretching, in one or more directions. The size and shape of the disperse phase particles, the volume fraction of the disperse phase, the film thickness, and the amount of orientation are chosen to attain a desired degree of diffuse reflection and total transmission of electromagnetic radiation of a desired wavelength in the resulting film. A substantial mismatch in refractive index between the continuous phase polymer and the disperse phase polymer along a particular axis has the effect that incident light polarized along that axis will be substantially scattered, resulting in a significant amount of reflection. By contrast, incident light polarized along an axis in which the refractive indices of the continuous and disperse phase polymers are substantially matched will be specularly transmitted or reflected with a much lesser degree of scattering. This effect is described in connection with a variety of embodiments, including both diffusely reflective polarizers and diffusely reflective mirrors. For these embodiments, the refractive index mismatch is the predominant factor relied upon to promote scattering. In comparison, the geometry of the particles of the disperse phase is said to have only a secondary effect on scattering.
The '816 Allen et al. patent also describes embodiments in which the first and second polymer materials are morphologically co-continuous.
The '543 Ouderkirk et al. patent describes the addition of dichroic dyes to the diffusely reflective polymer-based blend optical films. Dichroic dyes in combination with certain polymer systems exhibit the ability to polarize light to varying degrees. The dichroic dyes are able to absorb light of a particular polarization when they are molecularly aligned within the material. A higher dichroism ratio indicates a higher ability to polarize light.
U.S. Pat. No. 5,217,794 (Schrenk et al.) describes a lamellar polymeric film made of polymeric inclusions that are large compared with wavelength along two axes, disposed within a continuous matrix of another polymer material. The body thus includes discontinuous layers of at least one polymeric material within a matrix of another polymeric material which polymers differ in refractive index. The resulting multilayered lamellar polymeric body, depending upon the layer thicknesses selected, may reflect substantially white light and exhibit a silvery, metallic appearance, or may have bands of iridescent color.
U.S. Pat. No. 6,096,247 (Ulsh et al.) discusses embossing various types of optical polymer films, including films composed of blends of two or more polymeric materials of the type disclosed in the '543 Ouderkirk et al. patent. The heat source used by Ulsh et al. is said to soften the surface of the optical film rapidly enough to cause softening of the film surface without causing a significant change in the optical properties of the bulk film. In this way, the embossed optical films of Ulsh et al. exhibit reflection, transmission, absorption, and refraction characteristics in the bulk phase that are substantially the same as those exhibited by the optical film prior to embossing.